The invention relates to automated discrimination of cardiac events, and in particular, to cardiac events detected by an atrial electrogram (A-EGM).
In the medical fields of cardiology and electrophysiology, many tools are used to assess the condition and function of a patient""s heart from observed frequency, polarity and amplitudes of the PQRST complex associated with a heart cycle. One such tool is the electrogram (EGM), which is a form of an implantable cardiac monitor. An EGM may be included in devices such as a cardiac pacemaker, a pacemaker/cardioverter/defibrillator (PCD) or an implantable cardioverter/defibrillator (ICD).
An EGM that records the activity of an atrium of the heart is called an atrial EGM, or A-EGM. An A-EGM may detect arrhythmia in the atrium, such as bradycardia and tachyarrhythmia. Events detected by an A-EGM may be converted to electrical signals, which may be used in many ways. The signals may be relayed by telemetry to medical personnel for analysis and diagnosis, for example. The signals may be used for treatment by an implantable device, which has been programmed to detect and respond to a particular arrhythmia.
An A-EGM may also be used in concert with a ventricular EGM, or V-EGM. Medical personnel and implantable devices may use both A-EGM and V-EGM signals for diagnosis and treatment.
In diagnosis and treatment, it is important that the A-EGM provide a true representation of the atrial electrical activity. The A-EGM may, however, detect some ventricular electrical activity. In particular, an A-EGM may detect an R-wave associated with ventricular depolarization. An atrial sensing of ventricular depolarization is called a far field R-wave.
Ordinarily, the sense threshold of an A-EGM may be set lower than the sense threshold of a V-EGM. This is because the P-wave amplitude, which represents atrial depolarization, is significantly lower than that of the R-wave, which represents ventricular depolarization. Because an A-EGM is more sensitive than a V-EGM, an R-wave, whether intrinsic or triggered by a ventricular pace, may often be detected by an A-EGM. The R-wave detected by the A-EGM may have an amplitude exceeding the P-wave sense threshold.
Far field R-waves that are detected in A-EGM signals may lead to misinterpretation of atrial rhythms. In particular, a far field R-wave may suggest an event in the atrium, when in fact the far field R-wave results from a ventricular event. Sensing an R-wave in an A-EGM signal and mistaking the sensed signal for an atrial event is called xe2x80x9coversensing.xe2x80x9d Misinterpretation of atrial rhythms may in turn lead to an incorrect diagnosis and/or inappropriate treatment.
Several techniques have been put forward for rejecting far field R-waves and/or discriminating P-waves from far field R-waves. For example, U.S. Pat. No. 4,799,486 to DuFault describes a method and apparatus for suppressing the ventricular component of a signal detected by an atrial sensing lead, using an adaptive filter that employs the Widrow-Hoff least mean square algorithm. U.S. Pat. No. 4,799,493 to DeFault describes a tachyarrhythmia/fibrillation detector that employs the Widrow-Hoff least mean square algorithm to estimate a transfer function.
U.S. Pat. No. 4,825,870 to Mann et al. describes circuitry to detect and compensate for xe2x80x9ccrosstalk,xe2x80x9d which is defined a signal originating in one chamber of the heart being sensed by circuits designed to sense signals in the other chamber of the heart. If crosstalk occurs, a shortened atrio-ventricular (AV) delay is triggered.
U.S. Pat. No. 5,755,739 to Sun et al. illustrates methods and apparatus for discriminating atrial P-waves from ventricular events such as far field R-waves. The A-EGM signal is filtered with an adaptive filter and is subjected to a morphological analysis with respect to a morphological model of a P-wave called a xe2x80x9cP-wave template.xe2x80x9d
U.S. Pat. No. 5,759,196 to Hess et al. describes techniques for sensing far field R-waves and using the sensed far field R-waves to determine the presence of atrial tachyarrhythmia.
U.S. Pat. No. 5,778,881 to Sun et al. and U.S. Pat. No. 5,782,888 to Sun et al. describes the use of Hidden Markov Modeling techniques with wavelet transforms to discriminate cardiac events of interest in EGM signals. These techniques may be employed in connection with far field R-waves in A-EGM signals.
U.S. Pat. No. 5,814,083 to Hess et al. sets forth implementation of an algorithm that used sensed far field R-waves to determine whether to search for blocked 2:1 sensing.
U.S. Pat. No. 5,817,133 to Houben presents techniques for morphological filtering to eliminate far field R-waves from A-EGM signals. The morphological filtering employs morphological operations such as dilation and erosion operations and open and close operations.
Each of the above patents is incorporated herein in their respective entireties.
The invention is directed to techniques for filtering far field R-waves from A-EGM signals, resulting in accurate interpretation of atrial rhythms and delivery of appropriate therapies. Based upon the nature, rate and timing of atrio-ventricular events, a morphological model of an estimated far field R-wave or xe2x80x9ctemplatexe2x80x9d is selected from a plurality of templates. The estimated far field R-wave is then subtracted from the A-EGM signal, which includes the undesirable far field R-wave.
The difference is a filtered A-EGM signal in which the far field R-wave is attenuated. This filtered signal is indicative of atrial activity, and may be compared to a P-wave sense threshold. The risk that a far field R-wave will be mistaken for a P-wave is substantially reduced.
In one embodiment, the invention provides a method comprising receiving an A-EGM signal, selecting an estimated far field R-wave for a cardiac cycle from a plurality of estimated far field R-waves and subtracting the selected estimated far field R-wave from the A-EGM signal for the cardiac cycle. The selected estimated far field R-wave may be a function of cardiac events. The method may include, for example, selecting an estimated far field R-wave associated with a premature ventricular contraction when a premature ventricular contraction is sensed, or selecting an estimated far field R-wave associated with a ventricular pace when the patient receives a ventricular pace.
In another embodiment, the invention provides a device comprising an atrial lead and a filter that selects an estimated far field R-wave from a plurality of estimated far field R-waves and subtracts the selected estimated far field R-wave from an atrial electrogram signal received from the atrial lead. The plurality of estimated far field R-waves may be stored in memory in the device. The device may further include a comparator that compares the filtered atrial electrogram signal to a pre-selected atrial sense threshold and generates an atrial sense signal when the filtered atrial electrogram signal exceeds the atrial sense threshold.
The above summary of the invention is not intended to describe every embodiment of the invention. The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.